The main operating memory of virtually all modern desktop and laptop computers is implemented using dynamic random access memory (DRAM) components. DRAM is relatively inexpensive and provides excellent storage density relative to other types of semiconductor memory.
A defining characteristic of DRAM is that the individual storage cells in a DRAM component usually cannot hold their charge for more than a certain period of time, usually about 70 milliseconds. Consequently, to prevent loss of data, each cell in the DRAM component must be periodically sensed (read) and rewritten in a refresh operation. A refresh operation is thus a type of maintenance operation. Other types of maintenance operations, including calibration of the command interface of the memory device, also need to be performed from time to time. For instance, maintenance operations are needed for signaling optimization in both read and write directions.